Local drug applications to the ear are in widespread use and hold tremendous potential for the treatment of a spectrum of inner ear disorders. However, some drugs have pharmacokinetic properties that make them unsuitable for their intended purpose in the ear. Distribution along the cochlea is dominated by the rate of elimination to blood. Drugs that are rapidly eliminated do not spread far from the application site Dexamethasone, which is used as a therapy for cochlear disorders such as idiopathic sudden hearing loss, is a small, non-polar molecule that is rapidly eliminated from the perilymph (22 min half-time). This severely limits how far it spreads and would prevent therapeutic concentrations reaching speech frequency regions of humans. In contrast, the pro-drug dexamethasone phosphate, is polar, hydrophilic, and better retained in perilymph so it would be expected to distribute further along the cochlea. Similar challenges face many other drugs in use or in development. The projects in this grant will characterize the pharmacokinetic properties of a range of drugs and relate them to specific molecular properties (size, lipophilicity, polarity). In Aim 1 we will measure elimination rates for a range of steroids with different physical properties. This requires the use of novel techniques both to load perilymph uniformly with drug and to collect multiple, pure perilymph samples representing different regions of the ear. Steroids with the lowest elimination rates and therefore most appropriate for cochlear therapy will be identified. In Aim 2 we will establish entry routes and distribution of steroids following intratympanic applications. Recent studies have shown that some substances enter the vestibular perilymph via the stapes more readily than perilymph of scala tympani via the round window. Knowledge of where different drugs enter the ear is crucial to understanding distribution differences between auditory and vestibular regions. Spatial distribution of drugs following longer applications will be correlated with elimination rates to verify the dependence. Manipulations of drug entry rates at the round window and stapes by pharmacological treatments will be performed with the goal of optimizing drug entry specifically into the cochlea or vestibule. In Aim 3, we will similarly quantify elimination, entry routes and inner ear distribution for locally applied gentamicin and its fluorescent analog (GTTR). These studies will establish the presently unknown routes to vestibular and cochlear hair cells following local application and will determine whether the widely-used fluorescent analog has similar pharmacokinetics to the native drug. In Aim 4, accumulated knowledge of kinetic processes from the 3 experimental aims will be incorporated into a computer model that is made freely-available for others in the field to use. These projects will establish basic scientific principlesof inner ear pharmacokinetics derived from reliable measures with clinically-relevant drugs. The findings will impact local delivery in both animals and humans and help ensure that ineffective drug delivery is no longer a cause of treatment failure.